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Colloid: Electrokinetic properties

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1 Colloid: Electrokinetic properties
Physical Pharmacy 2 4/12/2017 Colloid: Electrokinetic properties Kausar Ahmad Kulliyyah of Pharmacy Physical Pharmacy 2 KBA

2 Contents Types of electrokinetic phenomena Measuring zeta potential
Traditional microelectrophoresis Laser Doppler velocimetry Applications Physical Pharmacy 2

3 Electrokinetic phenomena
Electrophoresis The movement of charged colloidal particles in electric field. Most practical. Electroosmosis When the charged solid surface is fixed, the electric field causes a movement of the liquid Streaming potential Forcing a liquid through a capillary or porous plug induces a difference of electric potentials Sedimentation potential Forced movement of charged solid particles in a liquid, e.g., due to gravitation induces a difference of electric potentials Physical Pharmacy 2

4 Potentials 0 - potential at charged surface
s - potential at Stern layer  - potential at plane of shear. Only zeta potential can be determined experimentally. Both 0 and s are thermodynamic and theoretical quantities and are calculated from theory only. Physical Pharmacy 2

5 Zeta potential The slipping/ shear plane separates the thin layer of liquid bound to the solid surface (elastic behavior) from the rest of liquid (normal viscous behavior). The electric potential at the shear plane is called zeta potential. From Physical Pharmacy 2

6 Determination of zeta potential
Measure electrophoretic mobility the electrophoretic mobility is the ratio of the velocity of particles to the field strength Physical Pharmacy 2

7 Traditional Measurement of Electrophoretic Mobility
An electrophoresis system consists of a capillary cell with electrodes at either end to which a potential is applied. Observe individual particles using a microscope and time their transit across a graticule. Physical Pharmacy 2

8 Smoluchowski equation
Physical Pharmacy 2 4/12/2017 Smoluchowski equation From Marian Smoluchowski: m=ez/h m electrophoretic mobility e electric permittivity of the liquid is the viscosity applies for thin double layer when the zeta potential is not too high large colloidal particles and high ionic strengths thin double layers (as compared with the particle radius): ka>>100 Physical Pharmacy 2 KBA

9 Huckel Equation m=ez/1.5h
Physical Pharmacy 2 4/12/2017 Huckel Equation size of particle is small compared to EDL (or thick EDL) Use Huckel equation: m=ez/1.5h Thickness of EDL, 1/κ When the size of the particle is smaller compared to 1/k (ka<<1) Physical Pharmacy 2 KBA

10 Complications in zeta potential determination
Physical Pharmacy 2 4/12/2017 Complications in zeta potential determination Electrophoretic retardation Relaxation effect Under electrical field, the particle and the charged cloud move in opposite direction. Thus becoming unsymmetrical and not spherical anymore. This non-uniform charge density results in electrophoretic retardation i.e. a reduction in the velocity of the migrating particle. This effect is accounted by Henry’s equation. Following electrophoretic retardation, the process whereby the moving particle reverts to a symmetrical spherical particle is known as relaxation. The relaxation effect is the distortion of the field induced by the movement of the particle. The extent depends on mobility and charge of counter ions. The effect was accounted for by Overbeek & Booth and later by Wiersema, Loeb & Overbeek Physical Pharmacy 2 KBA

11 Laser Doppler Velocimetry
Physical Pharmacy 2 4/12/2017 Laser Doppler Velocimetry Young’s interference fringes formed at stationary level Two coherent beams derived from the output of a low power helium-neon laser, are aligned at the stationary layer in the cell. At the crossing point of the beams (at the stationary layer), Young's interference fringes of known spacing are formed. Particles that are moving through the fringes under the influence of the applied electric field, scatter light whose intensity fluctuates with a frequency that is related to the particles velocity. The signals received from the scattered light are captured by a photomultiplier and analysed by a digital correlator. Using Fourier Transform, the correlator produces a frequency spectrum from which the mobility distribution and hence zeta potential are calculated. Physical Pharmacy 2 KBA

12 Fluctuation in intensity of scattered light
Physical Pharmacy 2 4/12/2017 Fluctuation in intensity of scattered light Fluctuation in intensity of scattered light is related to particle velocity. The higher the fluctuation, the higher the velocity, the higher the charge. From: Physical Pharmacy 2 KBA

13 Advantages of laser technique
Physical Pharmacy 2 4/12/2017 Advantages of laser technique High speed Applicable for non-aqueous, highly conductive, high ionic strength environment Can detect particles < 100 nm Does not differentiate particles i.e. even small ones are detected. Measures over thousands of particles Physical Pharmacy 2 KBA

14 Examples of Application
Physical Pharmacy 2 4/12/2017 Examples of Application To investigate the electrophoretic properties of blood lipid particles in connection to potential heart problems. Control of size and zeta potential of droplets of artificial blood, which is vital for its safe use. The relationship between the zeta potential of certain cells in amniotic fluid, and lung maturity (suggested by a study in Holland). Determination of IEPs are carried out to e.g. confirm the pH for flocculation/coagulation. Determination of CRC can be used to identify the nature of surface groupings e.g. sulfates, carboxylates etc Physical Pharmacy 2 KBA

15 Isoelectric point (IEP)
Physical Pharmacy 2 4/12/2017 Isoelectric point (IEP) If pH<4 or pH>8 there is sufficient charge to confer stability. If 4<pH<8 dispersion may be unstable. Most unstable at around pH 6 (IEP) From: Tutorials/Intro/pcs18.html Physical Pharmacy 2 KBA

16 Physical Pharmacy 2 4/12/2017 Zeta potential of chloramphenicol and glass particles in benzethonium (+) chloride solution. - an adsorbing cation Charge reversal concentration Irrespective of the charge of the particles, adsorbing ions will cover the surface of the particles and will influence the zeta potential. Non-adsorbing ions such as simple cations will change the zeta potential by changing the thickness of the double layer For adsorbing ions, depending on the surface groupings, the charge may reverse at a certain concentration. This is known as the charge reversal concentration (CRC). From: Florence & Attwood Physical Pharmacy 2 KBA

17 Physical Pharmacy 2 4/12/2017 References RJ Hunter, Zeta Potential in Colloid Science, Academic Press (1988) RJ Hunter, Foundations of Colloid Science Volume 2, Clarendon Press Oxford (1989) ID Morrison & S Ross, Colloidal Dispersions, Wiley-Interscience, New York (2002) Physical Pharmacy 2 KBA

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